A group build is a common theme that individual modelers adhere to. For instance a Pearl Harbor build would allow planes, ships & vehicles that are appropriate for that infamous day. Dioramas depicting the events may also be acceptable. Sometimes the GB is based on a certain scale, or it might be based on a certain kind of aircraft or ship, or even age group. GBs are normally time limited and are valid between a start and finish date. Typically a GB logo is created so that the participants can display it in their signature line.
All you have to do to participate is find one you would like to join, check to see if it's still open to join and build within the confines of the rules. I've never actually joined a GB as it requires a certain amount of discipline that I find sadly lacking in myself (ie actually finishing a model!). Perhaps somebody could start a "build anything, any scale, no time limit" GB?